"Call waiting" is a custom calling feature offered by most telephone companies, and can be subscribed to for a nominal monthly fee. With call waiting, a second call is announced by a soft "beep." Upon hearing this beep, the subscriber normally asks a current caller to wait while the second call is answered. The second call is answered by generating a "flash" signal, typically by briefly depressing the hook switch of the telephone which indicates the handset is momentarily on-hook. At the conclusion of the second call, the subscriber is returned to the first caller.
As an added feature, the subscriber can cancel call waiting to avoid interruption during an important call or during a data transmission with, for example, a facsimile machine or another terminal using a modem. To cancel call waiting, the subscriber normally must generate "*70" for tone dialers or "1170" for pulse dialers prior to starting a call. If a call is in progress, these codes must be preceded by a flash signal. This is feasible, for example, during voice transmission if the subscriber initiates the call and is available to generate the appropriate signal.
However, it is increasingly common to utilize a telephone line for both voice and data transmission. For example, a given line might have several telecommunication devices connected to it in parallel. In such a case, various devices such as telephone instruments, answering machines, facsimile machines, modems, meter interface communication circuits (alarm systems), video terminals, or personal computers, may share a common line. A user may desire to subscribe to call waiting for voice transmission applications, but still wish to use the line for data transmissions.
In such a situation, there is a risk that a data transmission will be interrupted by a call waiting signal which may corrupt the data transmission. This risk can be eliminated for outgoing calls by preceding a call with the appropriate code to turn off call waiting. However, when an incoming data transmission takes place which is received by a facsimile machine or modem, the call waiting tone will not be canceled. Thus, the risk remains that the transmission will be temporarily interrupted by the call waiting tone, and data will be lost. This risk is particularly acute when a data terminal is left unattended to receive important transmissions, such as facsimiles or other data transmissions.